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Old Sep 30, 2012, 11:35 AM
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sroge's Avatar
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LE Slats.. general layout question

Hi,

I have recently started building a new plane. Its emphasis will be on STOL and heavy duty landing gear including big inflatable tires, propeller clearance and strong fuselage/landing gear joint.

Wingspan will be around 85" with a chord of 15-16".

I still have to decide if I will use a Clark-Y or a USA-35B, or even mod one of them to get more lift.

I also decided to give each wing a 70% span flap with 20-25% chord depth.

My question:

Has anyone ever mounted a LE slat to a standard wing? Will a slat contribute to lift at all (Re around 200.000)?
Will a slat design result in a Clark-Y (or USA-35B) to which a slat is being mounted or will it rather be an Clark-Y (or USA-35B) overall outline with a "slot" after the LE?

When I take a look at an CH701 for example it seems to me it consists of some fat airfoil and a separate slat-wing attached to the LE.

Does anyone know where to find information on how to shape such a "slat-device" and how to find out where to mount it (distance to LE, angle, ...)

As I am quite a newbie I hope I found the right words to make my question understandable.

Stefan
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Old Sep 30, 2012, 12:00 PM
agnotology
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Both types of device, a slat (small airfoil ahead of the LE), and a slot, will add to the lift. The slats can have the advantage of being retratable, which might give lower drag when you don't want high lift.

They do seem to work at model Re.

The two classic papers are on the NASA server, as well as many others.

Slots:

http://ntrs.nasa.gov/archive/nasa/ca...1993091501.pdf

Slats:

http://ntrs.nasa.gov/archive/nasa/ca...1993084816.pdf

Kevin
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Old Sep 30, 2012, 01:29 PM
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Thank you.. the NACA documents seem to be an invaluable source of information concerning. I read a lot of them but obviously missed this one..
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Old Sep 30, 2012, 02:07 PM
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Slats CAN work - the catch is will they work noticably on the specific setup you are building .
Get the flaps sorted out in actual use to see how the wings works at various AOA and speeds
then, try adding your interpretation of slats. If they ar e itted closely they may simply add area OR too far out - may spoil everything
etc..

Years back there were some full scale types with spring loaded slats which came out if airspeed fell to a low point where LE pressure and pressure differences fell off .
Great idea as long as BOTH slats popped out at the same time --
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Old Sep 30, 2012, 02:51 PM
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The way you suggested is just what I intended: first finish the wings and install standard flaps. Then see what happens when I engage them. Maybe the plane will be slow enough. Another thing I will keep an eye on is maneuverability. I have an x-plane model of my plane and I managed to increase it by use aileron differential and in addition mixing the flap function into the ailerons the reverse wayŚlike a spoiler. With 60 degrees flaps down the ailerons go up some 5 degrees. At least in x-plane this made quite a difference.
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Old Sep 30, 2012, 03:50 PM
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That is a very controllable setup for some models
I have same on one of my planes - a Cub-
The sink rate goes very high - (AOA is really high) but the aircraft does not assume a nose high atitude - so the tail group is still works . a steep descent with solid control
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Old Oct 01, 2012, 03:28 PM
B for Bruce
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The trouble with slats as I see it is that the air doesn't scale directly. So if we take a slat shape and spacing from the airfoil as used on a full size aircraft and scale it directly to 1/4 or smaller it's quite possible that the air simply will not flow through and around it in the same manner.

So while slats can and do work we would need to have some way to test for the optimum shape for a model size to set the gap and find the slat width that works more optimally for our chosen model size.

However some other options do work well. The extended and drooped leading edge promoted by Francis Reynolds seems to work well at delaying the stall of the outer panels on his designs. Mind you there's also the idea that simply washing out the wing can do the same thing as well. Or possibly using an airfoil with a little more camber that is washed out such that the zero lift angles at the root and tips is matched. That has the possibility of creating a highly stall resistant low speed wing as well.

Going with a bit of "crow" like you describe with the ailerons reflexed up some 5 degrees is well worth experimenting with. If trying to "hang it up there" on the flaps you can still have the wing tips operating at a fairly high AoA and at that point adverse yaw can still be a possible issue depending on how the other in flight factors add up. The proof is in the pudding and it's worth trying it both ways at the proper "3.5 mistakes high" to learn what the model can do and what it can't.

Keep in mind too that all the high lift add-ons available take second place to starting with a low wing loading. If you truly want to fly slow, as opposed to just dragging the model to a halt so you can dive in and land in tight confines, then building light is the first line of attack.
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Old Oct 01, 2012, 07:09 PM
agnotology
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Bruce,

The NACA tests were done at about Re = 600k, and used on full size aircraft. The wind tunnel results are much closer to the models scale he is proposing, so should be quite applicable.

Kevin
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Old Oct 01, 2012, 08:05 PM
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Bit of a jump from 600K to 200K. And then there's the issue of how fussy the slats are to shape. The drawings in those papers are showing some fairly particular shapes and slat positions depending on the airflow to maximize their effect. Like in the difference in slot positioning for flaps deployed or retracted.

I'm not saying not to try. Not by a long shot. But I am suggesting that some ability to alter the position of the slats to allow testing and finding the optimum point would not be a bad option. And if the slats could be swapped to allow trying a few different choices for shapes that wouldn't hurt either.
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Old Oct 03, 2012, 08:06 AM
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Thinking about if slat designs may be transferrable to smaller scales at all without changing several parameters is quite an interesting aspect. And without any doubt keeping the weight low is also very important. At least in x-plane this "crow configuration" seems quite effective in maintaining aileron control up to high overall AoAs.

With Clark-Y airfoils parameters close to the Re=200.000 range this plane with 83"x16" wing and a weight of approx. 11 lbs it seems that controllable flight may be possible at 14-15 mph, depending on flap layout (plain, slotted, split). Unfortunately my only available engine for this size of an airplane is my FT-160, which is quite heavy.
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Old Oct 03, 2012, 11:38 AM
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I have experimented with slats (fixed) and slots for years, they are quite effective.
Years ago I modified a Great Planes"Spirit" glider. It was absolutely impossible to spin it with slots along @ 20% of half span from tip to root.
More recently I modified a Park Zone BF109G with fixed slats, much like the moveable slats on the full scale BF-109.
http://www.rcgroups.com/forums/showt...0#post14993828
I wold make the slat thinner than the NACA reports, they were @12% thick , make your slats thinner for the low Re we operate at.
PM me if I can be of any help.
Chris
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Old Oct 03, 2012, 11:50 AM
agnotology
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LE slats have been used successfully on lots of models previously. There are several examples in "RC Model Aircraft Design" by Andy Lemon, as well as in this forum. The shape doesn't seem to be that critical.

There are also lots of papers available on testing at low Re. They do work at least down to Re = 80,000:

http://airex.tksc.jaxa.jp/dr/prc/jap...9/63908009.pdf

Leading edge flaps are another alternative that also appear to work well at low Re, and might be easier to make retractable if that was desired:

http://www.stanford.edu/~nmb/researc...gEdgeFlaps.pdf

Kevin
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Old Oct 03, 2012, 05:18 PM
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I find that I've become overly picky about stuff like this over the past couple of months. I could likely even be accused of leaning towards being pedantic....

You guys are very right about the slats and slots for this. Even if they aren't perfect unless quite badly executed they will certainly provide the lion's share of whatever thay are capable of providing.
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Old Oct 12, 2012, 04:31 PM
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Vortex generators are another option, although I haven't seen these much on models either. Anyone out there try these? I've seen them on many full scale bush plane vids.
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Old Oct 17, 2012, 11:22 AM
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Mind you, it is possible to overdo things ... and find that your model will fly oh, so slowly, but that you instigate problems with control. For example, lack of airflow over the ailerons.

A rudder/elevator design might be worth considering, since the tail end should have the benefit of propwash and therefore remain effective for longer. At the very least, a generous dihedral should be incorporated ... for when the ailerons quit.
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